We have proposed that embryonic cells recognize one another and sort out according to species and/or cell type by non-random movement and by the formation of microvilli and the release of specific extracellular material(s) as specific responses to neighboring cells. We plan to study the formation of microvilli, microtubules, and microfilaments during the reaggregation of dissociated sea urchin embryonic cells by using transmission and scanning electron microscopy and by studying the effects of various microtubule and microfilament inhibitors. Hybrid embryos will be employed to study the role of paternal genes in the specificity of embryonic cell adhesion. We will further examine the role of extracellular material on the reaggregation process.